The present invention relates to radio frequency receivers, and more particularly to a method of reference frequency spur cancellation in synthesized measurement receivers.
Frequency spurs, i.e., frequency spectra unrelated to a received signal to be measured caused by sources within a radio frequency receiver, such as the reference frequency in a synthesized measurement receiver, are a constant problem to RF designers. Combining the effects of the reference""s high amplitude with its tendency to create many harmonics means that this signal may leak into the receiver""s front end, as shown in FIG. 1. This creates spurs when the receiver is tuned to any frequency near one of those harmonics. Up until now the hardware designer had to perfect the hardware""s shielding to the point that the spur is suppressed below the receiver""s noise floor.
The frequency of a spur caused by the reference frequency Fref at a first intermediate frequency stage in a synthesized measurement receiver is:
FIF1=nFrefxe2x88x92hFref=Fref(nxe2x88x92h)
where nFref=LO1 and hFref=spur. Since the second local oscillator is below the first IF frequency, at the second IF the spur frequency is:
FIF2=FIF1xe2x88x92mFref=Fref(nxe2x88x92hxe2x88x92m)
where mFref=LO2. h is an integer indicating the harmonic order of Fref, while m and n may be either integer or rational, i.e., the ratio of two integer numbers. In RF systems it is common for the signal input to an A/D converter to be in the frequency range between the Nyquist frequency, i.e., one-half the sample clock, and the frequency of the sampling clock itself. Therefore the frequency of the signal at the digital signal output is:
FDIF=Frefxe2x88x92FIF2=Fref(1xe2x88x92n+m+h)Hz
In terms of digital domain frequency the frequency of the digital signal is normalized by the reference frequency:
Fdigital=FDIF/Fref=(1xe2x88x92n+m+h)cycles/sample
What is desired is a method of canceling the spur in a frequency spectrum of a signal caused by a reference frequency used in a synthesized measurement radio frequency receiver.
Accordingly the present invention provides a method of reference frequency spur cancellation in a synthesized radio frequency receiver by determining values of an integer multiplier that cause the reference frequency to leak into the input signal path for known frequencies of the local oscillators derived from the reference frequency. For those integer values that cause such leakage, the amplitude and phase of the spurs are estimated in the digital domain and subtracted from the input signal. This removes signals from a narrow range of frequencies near spurs. To maintain the input signal at frequencies near spurs, the process is repeated with the reference frequency offset slightly, and the maximum values from the two measurement results provide the measurement of the input signal.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.